1. Field of the Invention
This is a continuation-in-part of U.S. patent application Ser. No. 07/187,723, filed Apr. 29, 1988, now U.S. Pat. No. 4,956,554, issued Sep. 11, 1990.
The present invention relates to heat exchangers. In particular, it relates to heat exchangers that make use of microwaves as the energy source.
2. Related Art
In general, heat exchangers are devices used to transfer heat from one heat conductive medium or source to another. The heat supplied from the medium to the heat exchanger may come from a variety of sources, for example, the burning of gas, oil, or coal. Another source of energy is electricity.
One source of energy that has been of interest in recent years is microwave energy. In a typical microwave heat exchanger, microwaves emitted from a microwave source are absorbed by a fluid carried within one or more microwave transparent pipes. The fluid heated by the absorbed microwave energy is then transported to the area to be heated by the fluid. The fluid may be used either to transfer heat indirectly, for example, by convection, or it may be used to directly transfer heat.
One consideration involved in the design of microwave heat exchangers is geometry. In order to allow for the efficient absorption of microwave energy, such heat exchangers are designed so as to allow the heat conductive medium a reasonable amount of exposure to the microwave energy. Representative examples of microwave heat exchanger configurations may be seen in the helical path used in U.S. Pat. No. 3,778,578 (Long et al.) and in the parallel paths used in U.S. Pat. No. 4,417,116 (Black).
The inventor has discovered that conventional microwave heat exchangers suffer from reduced efficiency due to the shadow created by the heat exchange medium (i.e., the fluid or gas within the microwave transparent pipes or conduits). Medium closer to the microwave source absorbs microwave energy and thus "shadows" the medium in the pipes at lower levels (i.e., further from the microwave source). The inventor has discovered that the lack of efficiency created by this "shadow" effect increases energy consumption, and necessitates the use of additional or larger capacity heating equipment. Such shadowing can be readily conceptualized by observing the geometry of parallel path and straight helical (cylindrical) heat exchanger.
Conventional microwave heat exchanges also suffer from another type of shadowing problem. The inventor has discovered that medium carried within any given level of the microwave-transparent pipe or conduit also has a tendency to "shadow" itself. That is, the portion of the medium which is carried closer to the microwave source tends to absorb the majority of the delivered energy. This absorption causes the medium on the side of the conduit closer to the source to become more excited than the medium on the other or farther away side of the same section of conduit.
The inventor believes that efforts to deal with this problem by merely reducing the inner diameter of the microwave transparent conduit frustrates the goal of maintaining the volumetric capacity of the microwave heat exchanger. Further, if parallel conduit sections are used to make up for loss in volumetric capacity, for example, the resulting structure may suffer from problems caused by the shadowing from pipe to pipe.
In order to operate, heat exchangers circulate or more the heat conductive medium from source to destination. In order to accomplish this movement of the medium, conventional microwave heat exchangers often use a mechanical pump. Typically, this mechanical pump is placed along the medium path and may be the only mechanism for circulation of the medium. Any mechanical pump exhibits a certain probability of mechanical breakdown. In addition to increasing hardware costs, such a mechanical pump may increase energy consumption of the system, thus reducing efficiency. A non-pump method of moving the heat conductive medium, that is both efficient and inexpensive, would be desirable.
As stated above, conventional microwave heat exchangers receive microwaves from microwave sources. A conventional microwave source contains a single magnetron unit. magnetron units are designed to operate over a safe operating temperature range. Operation outside the safe operating temperature range results in efficiency degradation and premature failure of the magnetron units. Thus, in applications which require a continuous supply of microwaves from the microwave source, the use of a single magnetron is inefficient and expensive if the magnetron unit is required to operate beyond its safe operating temperature range.
Microwave heat exchangers may be put to many uses or applications. It is known that microwave energy may be used in hot water heating applications. See, for example, U.S. Pat. No. 4,029,927. In this patent, for example, microwave energy applied to the entire volume of water in the hot water tank. Conventional devices which attempt to heat a large volume of water directly suffer from the deficiency caused by the absorption of microwave energy by the water that is close to the microwave source.